The present invention relates to a ferroelectric thin film, and more particularly to an SBTN ferroelectric thin film. The present invention also relates to the use of an SBTN ferroelectric thin film and a method for producing an SBTN ferroelectric thin film.
Recently, with the reduction of a sub-micron semiconductor device from VLSI to ULSI, the structure of a nonvolatile memory (NVRAM) is simplified to resemble a dynamic random access memory (DRAM) which consists of only one transistor and one capacitor. For this purpose, a thin film with ferroelectric properties is used as a component of the capacitor. Owing to the extremely high dielectric constant of a ferroelectric material, a ferroelectric random access memory (FRAM) cincluding a ferroelectric thin film as the component of the capacitor has advantages of reduced size, high speed, low operational voltage, low consumption power, low cost, and simple manufacturing procedure, compared to conventional NVRAMs.
At an early stage, lead zirconate titanate (PZT, i.e., Pb(ZrxTi1xe2x88x92x)O3) is a popular ferroelectric material. PZT, although exhibits remanent polarization, is subject to a fatigue effect. The fatigue effect results from the deficiency of oxygen in the structure of the PZT crystal. Although an oxide electrode can be used to solve this problem, the relatively high resistivity of the oxide electrode will render an undesired RC delay effect. Moreover, the oxide electrode is likely to produce a secondary phase which is non-ferroelectric during the growth of the PZT film. Therefore, the conditions for forming a PZT film is difficult to be controlled.
Recent investigations have examined the feasibility of alternative materials such as bismuth layered oxides having a perovskite-like layertype structure to substitute for the PZT film. For example, SrBi2Ta2O9 (SBT) is used as a fatigue-free material displaying nearly no change (less than 5%) in remanent polarization up to 1012 switching cycles with Pt electrodes. Owing to the characteristics of resistance to fatigue, these layered bismuth oxides are considered so far as the optimum material for NVRAM applications.
Commercialization of nonvalitile FRAM technology based on SBT, however, has been hampered by problems related to a high processing temperature ( greater than 750xc2x0 C.), low remanent polarization (2Pr), and low Curie temperature (Tc), thereby making the direct integration thereof into high density CMOS devices difficult. In general, the difficulty to reduce the post-deposition annealing temperature of SBT thin films was largely attributed to poor ferroelectric properties at an annealing temperature lower than 700xc2x0 C.
Boyle, U.S. Pat. No. 5,683,614 filed Aug. 16, 1996 and incorporated herein for reference, discloses a sol-gel type synthesis of SBT thin films wherein SBT is thermally processed in the presence of oxygen to impart ferroelectric properties to the resulting thin film. Perino et al., U.S. Pat. Nos. 5,426,075 filed Jun. 15, 1994 and 5,519,566 filed Mar. 14, 1995, and both incorporated herein for reference, disclose a two-sputtering-target method to form an SBT thin film at low temperature, and the SBT thin film is then thermally processed in the sputtering chamber to obtain ferroelectric properties. By observing the ferroelectric property data of the resulting SBT thin film, e.g., the remanent polarization (2Pr) of 10 xcexcC/cm2, coercive field (2Ec) of 100 kV/cm, it is apparent that the dielectric constant is a little too low and the operational voltage is a little too high.
Therefore, an object of the present invention is to provide an SBTN ferroelectric thin film with improved ferroelectric properties.
Another object of the present invention is to provide the use of a specified composition of SBTN ferroelectric thin film in a semiconductor device as a dielectric film.
A further object of the present invention is to provide a method for producing an SBTN ferroelectric thin film, which can be performed at a relatively low processing temperature.
According to a first aspect of the present invention, an SBTN ferroelectric thin film having a composition of SraBibTacNbdOx is provided, wherein a lies between 0.5 and 1, preferably, a=0.8; b lies between 2 and 2.7, preferably, b=2.5; c lies between 1 and 1.4, preferably, c=1.2; d lies between 0.6 and 1.1, preferably, d=0.9; and x lies between 8 and 10, preferably x=9xcx9c10.
According to a second aspect of the present invention, the use of a SraBibTacNbdOx ferroelectric thin film as a dielectric film in a semiconductor device is provided, wherein a lies between 0.5 and 1, preferably, a=0.8; b lies between 2 and 2.7, preferably, b=2.5; c lies between 1 and 1.4, preferably, c=1.2; d lies between 0.6 and 1.1, preferably, d=0.9; and x lies between 8 and 10, preferably x=9xcx9c10.
According to a third aspect of the present invention, a method for forming an SBTN ferroelectric thin film on a substrate is provided. The SBTN thin film has a composition of SraBibTacNbdOx, wherein a lies between 0.5 and 1, b lies between 2 and 2.7, c lies between 1 and 1.4, d lies between 0.6 and 1.1, and x lies between 8 and 10. The method according to the present invention includes steps of:
a) providing a first sputtering target formed of SreBifTagNbhOy above the substrate, wherein e lies between 0.5 and 1, f lies between 2 and 2.5, g lies between 1 and 2, h lies between 0.1 and 1, and y lies between 8 and 10;
b) providing a second sputtering target formed of Bi2O3 above the substrate; and
c) providing a first and a second radio frequency (RF) powers for the first and the second sputtering targets, respectively and simultaneously, to release target components from the two sputtering targets, which form the SBTN thin film on the substrate.
In an embodiment, the first and the second sputtering targets have different angles with the substrate, e.g. 90 and 60 degrees, respectively. The first RF power is varied from a first value, e.g. 70 W, to a second value, 120 W, with an increment of a third value, e.g. 10 W, for each time, and the second RF power is fixed at a fourth value, e.g. 20 W.
Preferably, the method further includes a post-annealing step after the step c) to transform the crystalline phase of the SBTN film. The post-annealing step is performed at a temperature ranged between about 500xc2x0 C. to about 700xc2x0 C., more preferably about 598xc2x0 C., a pressure selected from oxygen, nitrogen, argon and vacuum, and a pressure ranged between substantially 0 and about 10 mTorr within a period of time ranged between about 5 to about 60 minutes.